The present invention relates to an ultrasound diagnosis apparatus which extracts a harmonics component generated by non-liner ultrasound propagation from echo signal, and generates and displays an ultrasound image based on the extracted harmonics component.
The above method called a harmonic imaging method (HI) is a method which detects and images a harmonics component included in a signal from an ultrasound wave (for example, to transmit an ultrasound wave at 2 MHz and image at a high frequency of 4 MHz), and is developed for the purpose of more efficiently detecting an ultrasound contrast medium composed of micro bubbles.
Micro bubbles has a strong non-liner diffusion characteristic, and the diffusion signal thereof contains a large harmonics component in comparison with an organic tissue. Hence, by detecting only the harmonics component, fine blood perfusion which will be normally hidden in an echo from peripheral tissues (in case of using a base wave) can be imaged.
In recent years, attention has been paid to a Tissue Harmonic Imaging (THI) method, as another application of the Harmonic Imaging (HI) method. This method takes into consideration the image improvement effect of the Harmonic Imaging method, and therefore, a high-contrast B-mode image with reduced noise can be obtained for a so-called tough patient. Thus, this method is characterized in excellent rendering of an endocardium or the like.
In the Harmonic Imaging method, a non-linear "oscillation" signal of a contrast agent (of micro bubbles is imaged. On the other hand, the Tissue Harmonic Imaging method images a harmonics wave generated by so-called non-linearity of propagation in which a transmitted ultrasound wave is transferred being deformed.
Since the amplitude of this harmonics wave is proportional to the propagation distance of the ultrasound wave and square of the sound pressure of fundamental wave, and is therefore generated, concentrated on the central axis (i.e., in the region where the sound pressure is high). That is, it is possible to form a sharp ultrasound beam having a narrow main lobe and a low side lobe level, compared with the case of using a basic wave.
Since a beam having a narrow beam width and a low side lobe level can thus be formed in the Harmonic Imaging method, the resolution in the azimuth direction is improved by reduction of the beam width, and the contrast resolution is also improved by reduction of the side lobe level.
Although various superiority as described above has been predicted, various problems as follows have occurred when the Tissue Harmonic Imaging is carried out in practice. (1) The harmonics wave is not generated much effectively as expected. (2) The image quality is not much improved in the interested region where the ultrasound wave should have been converged. (3) When the normal imaging is switched to the Tissue Harmonic Imaging, the luminance level of an image greatly rises and drops so that the image is very difficult to watch. (4) If the region of interest is shallow from the surface of a human body, sufficient propagation distance cannot be maintained so that the harmonics component does not occur enough and the imaging is rendered difficult.